This invention relates to an optical interconnection apparatus that is capable of optical switching and/or multiplexing/demultiplexing functions.
Fiber optic communication is becoming ubiquitous. Advantages to using fiber optic communications are well known and each year more users require increasing bandwidth to transmit ever-increasing amounts of information. Unfortunately at present, the cost of installing, xe2x80x9claying,xe2x80x9d new fiber is prohibitive. Hence, it would be greatly advantageous to be able to pass greater amounts of information through existing fiber optic networks. Wavelength division multiplexing is an effective method of exploiting the large bandwidth of optical fibers. In addition to increasing the transmission capacity of a point-to-point link, wavelength division multiplexing is also becoming important in optical networks for routing and circuit switching.
Fiber optic networks require optical multiplexers-demultiplexers. A multiplexer-demultiplexer is capable of functioning as a multiplexer or a demultiplexer. A demultiplexer separates a single multi-wavelength beam of light, a spectrum of light, into a plurality of beams each comprising a component wavelength of the multi-wavelength beam of light, and a multiplexer combines a plurality of light beams having different wavelengths into a single multi-wavelength beam of light.
An optical wavelength multiplexer-demultiplexer has a passband. The passband is a portion of light having wavelengths between first and second limiting wavelengths that are transmitted with minimum relative loss. An optical multiplexer-demultiplexer is designed to have a minimum attenuation, or alternatively stated a maximum transmitence, for a particular wavelength band. Notwithstanding intended design, multiplexer-demultiplexers generally do not have a constant transmitence. The transmitence of the device is wavelength dependent and periodic. Often, there is an undulated effect to the output response within the passband. Flattening of the output response within the passband of wavelength demultiplexers is desirable because it relaxes the requirements on the wavelength control of optical sources.
Some known multiplexers do not efficiently provide a flat output response within their passbands. U.S. Pat. No. 5,412,744 entitled xe2x80x9cFrequency Routing Device Having Wide and Substantially Flat Passbandxe2x80x9d by Corrado Dragone, issued May 2, 1995, discloses a frequency routing device in which a flat output response within the passband is achieved by combining a routing device with an optical coupler. This document and all references therein are herein incorporated by reference. One limitation of this device, however, is an inherent loss of optical power due to the presence of the coupler.
U.S. Pat. No. 5,488,680 entitled xe2x80x9cFrequency Routing Device Having Wide and Substantially Flat Passbandxe2x80x9d by Corrado Dragone, issued Jan. 30, 1996, discloses a frequency, routing device in which a flat output response within the passband is achieved by coupling a first frequency routing device to a second frequency routing device. The output light from the first frequency routing device is launched into a second frequency routing device having a wavelength channel spacing equal to the free spectral range of the first device to provide a, substantially flat output response within the passband. A ripple in the substantially flat output response is generally undesirable, though present in the above mentioned devices. In addition, the solution disclosed in U.S. Pat. No. 5,488,680 does not lend itself to bulk optics.
Precise synchronism between the spectral responses of the first frequency routing device and the second frequency routing device as disclosed in U.S. Pat. No. 5,488,680 is required for effective implementation of the technique. It would be advantageous if it was possible to tune the spectral responses of the first frequency routing device and the second frequency routing device, which are cascaded together, independently. In the case where the two cascaded frequency routing devices are present on a same integrated chip, it would be difficult to tune the spectral responses of the two devices independently after fabrication. It is also noteworthy that in frequency routing devices such as Arrayed Waveguide Grating (AWG) devices employed by Dragone a fraction of power routed through the devices is diffracted into higher orders resulting in losses. Moreover, it is necessary to reduce crosstalk by blocking the optical power diffracted into the higher orders in a first Arrayed Waveguide Grating from entering a second Arrayed Waveguide Grating, as disclosed in an article in Technical digest Tuesday, Feb. 24, 1998, page 77, by Thompson, G. H. B. et al. For the aforementioned reasons implementation of the device of the devices disclosed in U.S. Pat. No. 5,488,680 are complex.
In addition, manufacture of integrated devices as disclosed in U.S. Pat. No. 5,488,680 is difficult because the integrated devices are lengthy and intricate. It would be advantageous to reduce ripple in the output response within the passband. It would also be advantageous to have a device that is manufactured economically, provides a substantially flat output response having reduced ripple over prior art devices within its passband. It would also be advantageous to reduce higher orders within the output response and to provide tunability of the device.
In accordance with the invention there is provided a wavelength multiplexer-demultiplexer comprising: a first routing device having a first input port for launching a multi-wavelength beam of light having at least n wavelengths into the first routing device, wherein n is greater than 1, and a first output port for exiting sub-beams generated by the first routing device from the multi-wavelength beam of light, and having an output response with a free spectral range; a second routing device having a second input port for receiving sub-beams exiting the first output port, a second output port, and a spectral response for providing a wavelength channel spacing, the second input port optically coupled to the first output port to provide a light coupling absent substantial mode mismatch.
The first routing device for providing a free spectral range that is approximately equal to the wavelength channel spacing provided by the second routing device.
According to an embodiment the first routing device comprises a resonant optical cavity having a forward and a rearward spaced apart reflective surface; the first input port for launching a multi-wavelength beam of light having at least n wavelengths into the resonant optical cavity is such that the beam is incident upon one of the reflective surfaces of the resonant optical cavity at an angle that is non-orthogonal to said reflective surface, the forward reflective surface being partially transmissive, so that a portion of the beam exits the forward partially transmissive surface as subbeams at a plurality of locations along the forward partially transmissive surface as the beam follows a zig-zag path between the forward and rearward surfaces.
In accordance with the invention there is further provided a wavelength multiplexer-demultiplexer comprising: a first routing device comprising a first port and a second port, the first routing device having an output response with a free spectral range, wherein a portion of light launched into the first port exits the second port at a location dependent upon a wavelength of the portion of light, a plot of location versus wavelength substantially defining a saw tooth function; and a second routing device comprising an input port and an output port, the second routing device having a spectral response with a channel spacing approximately equal to the free spectral range of the first device, wherein the input port of the second routing device is optically coupled to the output port of the first routing device for routing unguided light from the second port of the first routing device to the input port of the second routing device.